Formation of phases and microstructures in Al-8Si alloys with different Mg content

Abstract

Mg-containing high-silicon aluminum alloy is a heat-treatable aluminum alloy that is now widely used in the aerospace and automotive industries because of its high specific strength, high wear resistance and corrosion resistance, low thermal expansion coefficient, and low cost. More attention has been paid to optimizing the microstructure to increase the performance of this type of aluminum alloy. In the present work, the solidification processes of Mg-free and Mg-containing (0.33–1.32%) Al-8Si alloys were analyzed by the experimental results combined with the thermodynamic calculation. The results showed that α-Al, Si, and Al5 FeSi were in the Mg-free Al-8Si alloy ingots, while the Al5 FeSi phases in alloys with Mg additions were transformed into π phases (Al8 Mg3 FeSi6) by the reaction L+Al5 FeSi→α-Al+Si+Al8 Mg3 FeSi6. There was a binary eutectic reaction of L→α-Al+Al5 FeSi when the Mg content exceeded 0.51% and the Fe content was higher than 0.17%. With the increase of Mg content, the volume of Mg2 Si was gradually increased while the divorced eutectic phenomenon of the quaternary eutectic structure (α-Al+Si+Mg2 Si+Al8 Mg3 FeSi6) was weakened and the eutectic structure was significantly refined.